use warnings;
use strict;
use 5.010;

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

if ($line =~ s/\*([^*]+)\*\s*((?:$bagofwords)*)/*$1 $2* /) {
    say $line;
}

##</code><code>##

use v6;

token word { had | a | Sam | Tom  }

say '*Mary* had a little lamb'.subst:
        rx{ '*' (<-[*]>+) '*' \s+ [<word> \s+]+ },
        { '*' ~ join('_', $0, @($<word>)) ~ '* ' };