open FILE, 'test_file.txt' || die "$!\n";
my $text = join( '', @{ [ <FILE> ] } );
close FILE;

die "The file supplied is not of unix format!\n" if ( $text =~ /\r\n/ );


##</code><code>##

open FILE, 'test_file.txt' || die "$!\n";
while ( my $line = <FILE> ) {
  die "File is not of UNIX format!\n" if ( $line =~ /\r\n/ );
  last;
}
close FILE;