use strict;
use warnings;

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

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

print $output;