Hi there, it looks like you want to do this:

#!/usr/bin/perl

use strict;
use warnings;

my $file = '1140475.txt';;

open my $raw, '<', $file or die "open: $!";
open my $out, '>', "out_$file" or die "close: $!";

while ( my $line = <$raw> ) {
  chomp( $line );
  my @splits = split ( ',', $line );
  print $out "First line, first field: $splits[0]\n";
  last;
}

close $out or die "close: $!";

__END__
[download]

$ cat out_1140475.txt 
First line, first field: 1424621700
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But you should use a module if you can. A lot of people like Path::Tiny because it does the file opening, reading, and chomping, for you. In the example below I use split() on the first element of the array of lines returned by Path::Tiny::path() (there's actually only one line since I set the 'count' attribute), and then I assign the first element of the result of split() to $first.

#!/usr/bin/perl
use strict;
use warnings;

use Path::Tiny qw/ path /;

my $file = '1140475.txt';

my $first = ( split ',', (path($file)->lines( {chomp => 1, count => 1}
+ ))[0] )[0];

print "$first\n";

__END__
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$ perl 1140475-2.pl
1424621700
$
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