Thanks for your replies everyone. Here is what I now have: 

#!/usr/bin/perl
use warnings;
use strict;
use 5.010;

# The 3n + 1 problem (http://www.streamtech.nl/problemset/100.html)

my @pairs;
say "Input:";

while (<STDIN>) {
    if (/^[1-9]+\d*\s[1-9]+\d*$/) {    # check for and prevent illegal
+ entries
        chomp;
        push( @pairs, $_ );
    }
    else { say "invalid input!" }
}

say "Output:";

foreach my $pair (@pairs) {
    my ( $first, $second ) = split( / /, $pair );
    my $max_cycle_length = 0;
    if ( $first > $second ) {
        ( $first, $second ) = ( $second, $first );
    }    # doesn't matter whether the higher number is entered first o
+r second

    foreach my $num ( $first .. $second ) {
        my $cycle_length_out = totalizer($num);
        if ( $cycle_length_out > $max_cycle_length ) {
            $max_cycle_length = $cycle_length_out;
        }
    }
    say "$first $second $max_cycle_length";
}

sub totalizer {
    my $number = shift;
    my $current_cycle_length;
    for ( $current_cycle_length = 1 ; $number != 1 ; ++$current_cycle_
+length ) {
        if ( $number % 2 != 0 ) {
            $number = ( $number * 3 + 1 );
        }
        else {
            $number /= 2;
        }
    }
    return $current_cycle_length;
}

[download]

I hope this is an improvement.

In reply to Re: General style advice requested by iangibson
in thread General style advice requested by iangibson

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