The "." metacharacter matches all characters except any newline characters in a multi-line string. If the "s" option of "m//s" is used, embedded newline characters will also be matched. This is convenient to match a pattern in multiple lines.

    #! C:\programfiles\perl\bin\perl
    print "content-type: text/html\n\n";
    if ("a\nb\nc" =~ m/a.b/)
     {
      print "matched the string a\nb\nc!\n";
     } 
    else 
     {
      print "no match in the string a\nb\nc!\n";
     }
    print "<br>";
    if ("a\nb\nc" =~ m/a.*c/s)
     {
      print "matched the string a\nb\nc!\n";
     }
    else 
     { 
       print "no match in the string a\nb\nc!\n";
     }

     no match in the string a b c!
     matched the string a b c! 

In the above example the string "a\nb\nc" is matched with the pattern m/a.*c/s, since the newline characters are present in between the pattern it gives a mismatch as m// does'nt match newline characters, but when the "s" option is used even if the embedded newline characters are present pattern is matched.