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use strict;

...
my $output = "<keyword>" . join ("</keyword>, <keyword>", split /\s*(?
+:,|;|\&mdash;|&ndash;)\s*/, $input) . "</keyword>";

print $output;

use strict;
use warnings;
...
}

print $output;

while( my($val, $sep) = splice @data, 0, 2 )
{
    $sep = "" unless $sep;
    $output .= "<keyword>$val</keyword>$sep";
}