my @mail = mail_lesen();
my @nachricht; # nachricht ohne Header

my $im_header = 1;
while (defined(my $zeile = shift(@mail))) {
    if ($im_header) {
        if ($zeile eq '') {
            $im_header = 0;
        }
        next;
    }

    push @nachricht, $zeile;
}

my $nachricht = '';
{
  local $/ = "\n\n";
  open(my $fh,"<",$mail) or die $!;
  while(my $part = <$fh>){
    $nachricht .= $part unless($. == 1);
  }
  close $fh;
}

print $nachricht;

if($line =~ /^$/){
   print "Leerzeile\n";
}

if (length $zeile == 0) {...}

if ($zeile eq '') {...}

if (length $zeile == 0) {...}

if ($zeile eq '') {...}

#!/usr/bin/perl

use strict;
use warnings;

while(my $line = <DATA>){
    print $.,":", length($line),"\n";
}

__DATA__
Diest ist

C:\>mail.pl
1:10
2:1

#!/usr/bin/perl

use strict;
use warnings;

#local $/ = "\n\n";
while(my $line = <DATA>){
    print $.,":", length($line),"\n" unless($line eq '');
}

__DATA__
Diest ist

C:\>mail2.pl
1:10
2:1