if ($sentence =~ /foo/gis && $sentence =~ /bar/gis)
{
   # do something further with $sentence 
}

##</code><code>##

if ($sentence =~ /foo/gis)
{
   if ($sentence =~ /bar/gis)
   {
      # now do something with the sentence
   }
}

##</code><code>##

if ($sentence =~ /foo/gis)
{
    my $final_test = $sentence;
    if ($final_test =~ /bar/gis)
    {
       # *now* do something with the sentence...
    }
}