If you mean that you only want the match to succeed if a string doesn't contain a substring you could use a negative look-ahead.

my $string = q{the quick black fox};
my $substring = q{brown};
if ( $string =~ m{\A(?!.*\Q$substring\E).*fox} )
{
    print qq{Found a non-$substring fox\n};
}
else
{
    print qq{Match failed, $substring seen\n};
}
[download]

I hope this is of use.

Cheers,

JohnGG

Update: Typo, s/negatine/negative/