#!/usr/bin/perl -w

use strict;

my $filename = <>;
chomp $filename;
my @filenam = split ('\.', $filename);
my $fi;
my $fff=($filenam[0],"_header",".txt");
open($fi,">", $fff) or die "Open failed for $fff: $!";

__END__

##</code><code>##

C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification>perl filenameTransmogrification.pl
Useless use of array element in void context at filenameTransmogrification.pl line 9.
Useless use of a constant in void context at filenameTransmogrification.pl line 9.
abc.dat

C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification>dir
 Volume in drive C has no label.
 Volume Serial Number is E88F-F456

 Directory of C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification

07/17/2013  09:52 AM    <DIR>          .
07/17/2013  09:52 AM    <DIR>          ..
07/17/2013  09:52 AM                 0 .txt
07/17/2013  09:51 AM               232 filenameTransmogrification.pl
               3 File(s)            466 bytes
               2 Dir(s)  78,355,476,480 bytes free

##</code><code>##

my $fff=($filenam[0],"_header",".txt");
    to
my $fff=$filenam[0] . "_header" . ".txt";

##</code><code>##

C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification>perl filenameTransmogrification2.pl
abc.dat

C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification>dir
 Volume in drive C has no label.
 Volume Serial Number is E88F-F456

 Directory of C:\Steve\Dev\PerlMonks\P-2013-07-17@0950-Filename-Transmogrification

07/17/2013  09:53 AM    <DIR>          .
07/17/2013  09:53 AM    <DIR>          ..
07/17/2013  09:52 AM                 0 .txt
07/17/2013  09:53 AM                 0 abc_header.txt
07/17/2013  09:51 AM               232 filenameTransmogrification.pl
07/17/2013  09:52 AM               234 filenameTransmogrification2.pl
               4 File(s)            466 bytes
               2 Dir(s)  78,354,427,904 bytes free