#!/usr/bin/perl -w
use strict;

use Fatal qw(open close);

my @files= @ARGV;

my %fh; # file => file handle
foreach my $file (@files)
  { open( my $fh, '<', $file);
    $fh{$file}= $fh;
  }

while( keys %fh)
  { foreach my $file (@files)
      { if( exists $fh{$file})
          { my $fh= $fh{$file};
            if( defined( my $line= <$fh>))
              { chomp $line; print $line;} # regular line
            else
              { delete $fh{$file};       } # eof reached for this file
          }
      }
    print "\n"; # end of line for all files
  }