The simple way:

#! perl -slw
use strict;

open FH, '<', $ARGV[0] or die "file not found";

my $line = 0;
my %hash
;
while( <FH> ) {
    $hash{ $line } = undef; #6ht line
    $line++;
}
close FH;

print `tasklist /nh /fi "PID EQ $$"`;

__END__
c:\test>junk3 junk.dat

perl.exe                      1128 Console                    1     29
+,176 K
[download]

That said, your code makes no sense. You are keying a hash with consecutive numbers, why not use an array? But then your values are undef, what is the point?