@array = ( "foo bar", "bar foo" );
@array2 = ( "foo", "bar" );
$string = "there is a foo bar and a bar foo and also foo and bar.";

print "string before - $string\n";

$swapString = join ("|", @array, @array2);
print "swapString 1 - $swapString\n";
$string =~ s/($swapString)/<B>$1<\/B>/gi;

print "string after first - $string\n";

##</code><code>##

string before - there is a foo bar and a bar foo and also foo and bar.
swapString 1 - foo bar|bar foo|foo|bar
string after first - there is a <B>foo bar</B> and a <B>bar foo</B> and also <B>foo</B> and <B>bar</B>.

##</code><code>##

$swapString = join ("|", sort { length($b) <=> length($a) } @array, @array2);