$text= q/foo bar foo/;
($tmp = $text) =~ s/\bfoo\b//g;
print $tmp,"\n";

($tmp = $text) =~ s/(?<!\w)foo(?!\w)//g;
print $tmp,"\n";

($tmp = $text) =~ s/(?<=[^\w])foo(?=[^\w])//g;
print $tmp,"\n";

# which prints:
 bar
 bar
foo bar foo