my $q = 'foo and not (bar or beer)'; #user input
$q =~ s/(^|[^\\])\//$1\\\//g; #escape slashes
my $searchexpr = join ' ', map {s/\/(and|or|not)\/si/$1/gi;$_} map {s/([^\(\)]+)/\/$1\/si/g;$_} split /\s+/, $q;
$searchexpr =~ s/\/si (not )?\//\/si and $1\//g;
# $searchexpr now is: /foo/si and not (/bar/si or /beer/si)

my $found = 0;
foreach my $d (@files) {
  open F, $d;
  my @f = <F>;
  $_ = join '', @f;
  $found++ if eval $searchexpr;
  close F; }

print 'The expression was true in '.$found.' files';