use strict;
use warnings;

my $line='*Mary* had a little lamb';
my $bagofwords='had |a |Sam |Tom ';

$line = join ' ', map {s/(\*.*\* | ^(?:$bagofwords)$)/*$1*/x; s/\*\*/*/g; $_}
    $line =~ /(\S+)/g;
$line =~ s/\* \*/_/g;

print $line;

##</code><code>##

*Mary_had_a* little lamb