perltidy can help make this type of error more obvious by auto-indenting your code:

#!/usr/bin/perl
use strict;
use warnings;
use List::Util qw(sum);

my @list_of_numbers = ( 1 .. 50 );
my $i               = ();
my $a               = ();

sub mean {
    return sum(@_) / @_;
}

sub above_mean {
    $i == mean(@list_of_numbers);
    foreach (@list_of_numbers) {
        if ( $a > $i ) {
            print "$a is above mean, which is mean(@list_of_numbers)";
        }

        print above_mean(@list_of_numbers);
[download]