You could put your entire locker list into an array, put the leased lockers that you harvested from the DB into a hash, then generate a new array for locker numbers that have not been leased.

use warnings;
use strict;

my @lockers = (1..25);

my %leased = map {$_ => 1} (1..12, 14..25);

my @avail = grep {! defined $leased{$_}} @lockers;

print "$_\n" for @avail;

__END__
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