Use File::Copy.

use File::Copy;
use strict;

my($sourcedir) = shift || die "No source dir";
opendir(DIR,"$sourcedir");
foreach (grep !/^\.\.?$/,readdir DIR) {
    if (/^(\S+)_(\S+)\.(\S+)$/) {
    if (move($_,"$1-$2.$3")) {
        print "Moved $_ to $1-$2.$3\n";
    } else {
        print "Error moving $_ to $1-$2.$3\n";
    }
    }
}
[download]

HTH,
Shendal