my @dirlist;
print qq(Enter a valid directory);
chomp (my $ans = <STDIN>);
while ( $ans ne "" ) {
   if ( $ans =~ m/foo/g ) {
       push @dirlist, $ans;
   } else { 
       print qq(That is not a valid dir);
   }
   print qq(Enter a valid directory);
   chomp ($ans = <STDIN>);
}
# @dirlist is now filled happily