Assuming that you've already used CGI.pm and declared the hash. And also that the text field is called hashdata...

foreach (split /,\s*/, param('hashdata')) {
  my ($k, $v) = split(/=>/);
  $hash{$k} = $v;
}
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If you know the hash is going to be empty at the start, then it gets a bit simpler...

%hash = map { split /=>/ } 
        split /,\s*/, param('hashdata');
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You might also want to add a little data filtration on the keys and values to at least strip leading and trailing whitespace otherwise "a=>4" will generate a different key to say " a =>4". Something like:

foreach (split /,\s*/, param('hashdata')) {
   my ($k, $v) = split(/=>/);
   whitespace(\$k,\$v);
   $hash{$k} = $v;
}


sub whitespace {
   while (my $ref = shift){
      $$ref =~ s/^\s+|\s+$//g
   }
}
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We pass the sub references to the variables so we can change them in the sub and not bother having to return the changed values and assign them.

cheers

tachyon