use strict;

my $input  = "keyword1, keyword2; keyword3— keyword4 – keyword5";
my $output = "<keyword>" . join ("</keyword>, <keyword>", split /\s*(?:,|;|\&mdash;|&ndash;)\s*/, $input) . "</keyword>";

print $output;

##</code><code>##

use strict;
use warnings;

my $output = "";
my $input  = "keyword1, keyword2; keyword3&mdash; keyword4 &ndash; keyword5";
my @data   = split /\s*(,|;|\&mdash;|&ndash;)\s*/, $input;

for (my $i=0; $i<=$#data; $i+=2)
{
    $output .= "<keyword>" . $data[$i] . "</keyword>" . ($data[$i+1] ? $data[$i+1] : "");
}

print $output; 

##</code><code>##

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