Like this?

use strict;

my $input  = "keyword1, keyword2; keyword3— keyword4 – key
+word5";
my $output = "<keyword>" . join ("</keyword>, <keyword>", split /\s*(?
+:,|;|\&mdash;|&ndash;)\s*/, $input) . "</keyword>";

print $output;
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Or do you want to preserve the original separator in your output?

Update: separator preserving version:

use strict;
use warnings;

my $output = "";
my $input  = "keyword1, keyword2; keyword3— keyword4 – key
+word5";
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;
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Alternative inspired by bart:

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