Loops in Perl

while Loops

The while loop is a flexible part of perl that allows you to either loop while a condition is met, or to loop over things available through the diamond operator (<>), like the output of a filehandle.

Basic while loop with a counter:

my $i = 0;
while ($i < 5) {
    print "i is $i\n";
    $i++;
}

Using a filehandle/diamond operator loop to loop over input lines from a file, stdin, or the diamond operator:

open(my $fh, '<', 'file.txt') or die $!;
while (my $line = <$fh>) {
    chomp($line);
    print "Line: $line\n";
}
close $fh;

You can also just rely on perl's default variable $_ for this:

while (<$fh>) {
    chomp;
    print "Line: $_\n";
}

If you're reading from STDIN or piped input:

while (<STDIN>) {
    print "You typed: $_\n";
}

for Loops

You can use for loops just like in C.

for (my $i = 0; $i < 10; $i++) {
    print "Current iteration: $i\n";
}

You can also use for loops to loop over an array.

my @arr = ('one', 'two', 'three');
for my $item (@arr) {
    print "Item: $item\n";
}

When iterating over arrays or lists, using for (rather than foreach) is considered more idiomatic perl.

You can also omit the variable declaration and use the default variable $_:

my @arr = ('one', 'two', 'three');
for (@arr) {
    print "Item: $_\n";
}

foreach Loops

for is just an alias for foreach, so this is also valid.

This is the type of loop that you want to use over an array.

my @arr = ('one', 'two', 'three');
foreach my $item (@arr) {
    print "Item: $item\n";
}

You can also omit the variable declaration and use the default variable $_:

my @arr = ('one', 'two', 'three');
foreach (@arr) {
    print "Item: $_\n";
}

Looping over Ranges

Perl can loop over numeric ranges directly, with similar syntax to Bash.

for my $n (1..5) {
    print "Number: $n\n";
}
# or
for (1..5) {
    print "Number: $_\n";
}

Same 1..5 syntax to bash, but with parentheses instead of braces.

Perl Equivalents of continue (next) and break (last)

The continue function doesn't work the same way as in Bash or other languages.
In Perl, continue is part of the given/when syntax (kind of like switch/case).

What you want to use, to continue to the next iteration of the loop, is next.
The next function is the equivalent of continue in Bash/others.

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If you want to break out of a loop, use the last keyword.
This will break out of the loop and continue executing the program.