Perl Control and Conditional Statements,
Statement Modifiers
(and Conditional Operator)

elsif

else

unless

while

In Perl conditionals and loops are defined in terms of a block rather than a statement (as in C) so curly braces are always required.

Conditional Control Statements


if(cond) {
    # if cond is true evaluate statements in block surrounded by curly braces,
    # curly braces are required
}


if(cond) {
   # stuff
} else {
   # more stuff
}


if(cond) {
} elsif(cond2) {
} elsif(cond3) {
...
} else {
}


unless(cond) {
    # if !cond is true evaluate statements in block surrounded by curly braces
    # no extra clause (elsunless)
}

Unconditional Control Statement (actually function)

goto LABEL # example #    label:
goto EXPR  # an expression that evaluates to a label name
goto &NAME

One can construct a FORTRAN like computed goto as follows:
goto ("FOO", "BAR", "JUNK")[$i];
(May not work in compiled perl).

Loop Control Statements


while(cond) {
    # repeat block while cond is true
}


until(cond) {
    # repeat block while !cond is true
}

while(@ARGV) {
    process(shift @ARGV); # fifo pop
    # loop terminates when @ARGV is exhausted
}


do BLOCK # executes commands in block, returns last expression value
####
# when modified by a loop modifier the block
# is executed once before testing the condition.
####

do SUBROUTINE(LIST) # deprecated form of subroutine call

do EXPR # evaluates EXPR as a filename and executes
# the contents of the file as a Perl script. (See @INC array).



for(init; test; step) {
    # where init test step are expressions, like in C
}


foreach $user (@users) {
    # This block is executable for each element of array @users (or more
    # generally over any list), where loop variable $user is a reference 
    # to the element, not a copy.
    #
    # Note1 the the loop variable, $user in this case, is implcitly local
    # to the loop, overriding a more global variable of the same name,
    # e.g. declared with my().
    #
    # foreach is actaully an alias for for.
}

foreach $key (sort keys %hash) {
    # loop over sorted keys of a hash.
}

Loop Control Commands

# a loop (or any statement) can be labeled and referered to:
LINE: while($line=) {
    last LINE if $line eq "\n"; # stop on first blank line
    next LINE if $line =~ /^#/; # skip comment lines
    ...
}

next; # inside a loop skips to end of current loop (C: continue;)
next LABEL; # continue with next iteration of loop with LABEL

last; # inside a loop leaves current loop (C: break;)
last LABEL; # leave loop with LABEL

redo; # restart a loop without doing test or step
redo LABEL; # restart loop with LABELwithout doing test or step

A BLOCK by itself (labeled or not) is semantically equivalent to a loop that executes once. Thus you can use last to leave the block or redo to restart the block. A next will also exit the once-through block but will execute a continue block, while a lastdoes not.

There is no case or multiway switches in Perl, but you can construct a labelled block that looks like it:

SWITCH: {
   if(/^abc/) { $abc=1; last SWITCH; }
   if(/^def/) { $def=1; last SWITCH; }
   if(/^xyz/) { $xyz=1; last SWITCH; }
}

Simple Statement Conditional Modifier

Any simple statement may optionally be followed by a single modifier just before the terminating semicolon (or block ending). The possible modfiers are: