This is working well.So I am getting $str="himanshu.txt"

Not possible, $2 is always undefined.

#!/usr/bin/perl --
use strict;
use warnings;
my $str="himanshu.txt^@";
if($str =~ /(\w+)/) {
    $str = "$1.$2";
}
print "$str\n"
__END__
Use of uninitialized value in concatenation (.) or string at - line 6.
himanshu.
[download]

What will the regular expression?
use re 'debug'; to see, ex

#!/usr/bin/perl --
use strict;
use warnings;
use re 'debug';
my $str="himanshu.txt^@";
if($str =~ /(\w+)/) {
    $str = "$1.$2";
}
print "$str\n"
__END__
Compiling REx `(\w+)'
size 7 Got 60 bytes for offset annotations.
first at 4
   1: OPEN1(3)
   3:   PLUS(5)
   4:     ALNUM(0)
   5: CLOSE1(7)
   7: END(0)
stclass "ALNUM" plus minlen 1
Offsets: [7]
        1[1] 0[0] 4[1] 2[2] 5[1] 0[0] 6[0]
Matching REx "(\w+)" against "himanshu.txt^@"
Matching stclass "ALNUM" against "himanshu.txt^@"
  Setting an EVAL scope, savestack=5
   0 <> <himanshu.txt>    |  1:  OPEN1
   0 <> <himanshu.txt>    |  3:  PLUS
                           ALNUM can match 8 times out of 2147483647..
+.
  Setting an EVAL scope, savestack=5
   8 <manshu> <.txt^@>    |  5:    CLOSE1
   8 <manshu> <.txt^@>    |  7:    END
Match successful!
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 7.
himanshu.
Freeing REx: `"(\\w+)"'
[download]

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