my $string = "ABCDEFGHIJKLMNOPCDEFQRST";
my $find = "CDEF";

if( $string =~ /$find/ ){
  print "found $find in $string\n";
}
# OR
my $index = index( $string, $find );
if( $index > 0 ){
  print "found $find in $string at position $index\n";
}