#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $tokens = "32,15,4,72,13,28,14";
my @tokens = (split/,/,$tokens);
my %tokens = map {$_ => 1}@tokens;

print Dumper (\%tokens);

__END__

Prints:
$VAR1 = {
          '4' => 1,
          '32' => 1,
          '28' => 1,
          '72' => 1,
          '13' => 1,
          '14' => 1,
          '15' => 1
        };

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $tokens = "32,15,4,72,13,28,14";
my @tokens = (split/,/,$tokens);
my $i=0;
my %tokens = map {$_ => $i++}@tokens;

print Dumper (\%tokens);
__END__
Prints:
VAR1 = {
          '4' => 2,
          '32' => 0,
          '28' => 5,
          '72' => 3,
          '13' => 4,
          '14' => 6,
          '15' => 1
        };

##</code><code>##

my @tokens = qw (32 15 4 72 13 28 14);
my $i=0;
my %tokens = map {$_ => $i++}@tokens;