elements from array @a that are not in array @b

The more idiomatic way to do this is with a lookup hash. The combination of for/grep is actually two nested loops, meaning that you do a*b comparisons. A hash lookup will be faster for most situations, and clearer to anyone with much Perl experience. To show elements of @a not in @b:

#!/usr/bin/env perl
use Modern::Perl;

my @a = qw/ foo bar baz /;
my @b = qw/ foo bar biz /;

my %b = map { $_ => 1 } @b;

for (@a){
    say unless $b{$_};
}
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