You can't use exists in this case, as you are not looking to see if a hash key exists, you need to check within the array to see if an element exists. Here's an example of what I think you want to achieve:

#!/usr/bin/perl  

use strict;
use warnings;
use Data::Dumper;

use List::Util qw( first );

my @array1 = ( "proc1", "proc2", "proc3", "proc4" );
my @array2 = ( "proc1", "proc2", "proc3", "proc4", "proc5" );

my %HoA = (
            prebuilds  => [@array1],
            postbuilds => [@array2],
);

for my $postbuild ( @{ $HoA{ postbuilds } } ){

    print "$postbuild: ";

    if ( first { $_ =~ $postbuild } @{ $HoA{ prebuilds } } ){
        print "yes\n";
    }
    else {
        print "no\n";
    }
}
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