I am unable to reproduce your problem. This SSCCE never prints any output for me.

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

my @result = 'whew';
my %master_letter_freq = (
    w => 1,
    h => 1,
    t => 1,
    o => 1,
    r => 2,
    e => 1,
);

OUTER: foreach (@result) {
    my %seen;
    $seen{$_}++ for (split //,lc $_);
    foreach (keys %seen) {
        next OUTER if ($seen{$_} > $master_letter_freq{$_});
    }
    print "$_\n";
}
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