You can use a hash to keep track of each letter by setting the value of each key to the number of times it appears in the word and then decrementing that value each time that letter is removed from @letters. Code below creates a new array to hold the remaining letters - probably not the most efficient way but may help:

use strict;
use warnings;

my @letters = qw(a a a b b b c c c);
my @word = qw(a a a b b);

# create the hash
my %count;
for my $letter (@word) { $count{$letter}++ }

my @remain;
for my $letter (@letters) {
    if (not $count{$letter}) {
        push(@remain,$letter);
    }
    else {
        $count{$letter}--;
    }
}
print "@remain\n";

# prints: b c c c
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