try defining slotarray as a hash, which is how you are trying to use it in the assignment. Then you need to access it as a hash, not an array, when you try to print to outfile. If the problem is that $1 is empty then your regexp is not matching, and i suggest tweaking that.

#open files here
my $counter = 0;
my %slotarray;

while ( <DPSLOTFILE> ) {
    my $dpdrivloc = "MSL6000 Trinity";
    if ( /\Q$dpdrivloc\E:\s*(\d+)/ ) {
        $slotarray{$counter} = "$1";
        #print OUTFILE "$1";
        $counter++;
    }
}

foreach my $key (keys %slotarray) {
    print OUTFILE "$key: $slotarray{$key}\n";
}
#close files here
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