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

my @slot;
my $dpdrivloc = "MSL6000 Trinity";
my $regex     = qr/\Q$dpdrivloc\E:\s*(\d+)/;

#  ... open DPSLOTFILE ...

while ( <DPSLOTFILE> ) {
    if (my ($numb) = $_ =~ $regex) {
      push @slot, $numb;
    }
}

print Dumper \@slot;