in reply to Loop through 2 files in parallel

You definitely don't want to nest your file-reading loops. Instead, try something like this (untested): 
while (my $w1 = <FILE> and my $w2 = <FILE2>) {
  if ($w1 lt $w2) {
    print "\n";
    $w1 = <FILE>;
  }
  elsif ($w1 gt $wt) {
    print "\n";
    $w2 = <FILE2>;
  }
  else {
    print $w1;
  }
}
# Leftover lines in either file?
print "\n" while <FILE>;
print "\n" while <FILE2>;

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Re^2: Loop through 2 files in parallel
by rwitmer (Initiate) on Nov 10, 2010 at 21:07 UTC
    By george, I think this will work. You are truly a monktastic monk! Thanks!
[reply]
      at least one error in previous code ... while condition is wrong. 
      
use strict;
use warnings;

my @FILES;

open $FILES[0], '<', 'file1' or die;
open $FILES[1], '<', 'file2' or die;

my @w = map scalar <$_>, @FILES;

my $empty = "blank line\n";
my %map = (
    -1 => [ 0 ],
    1  => [ 1 ],
    0  => [ 0, 1 ],
);

while (defined $w[0] && defined $w[1]) {
    my $cmp = $w[0] cmp $w[1];

    print $cmp ? $empty : $w[0];
    @w[ @{$map{$cmp}} ] = map scalar <$_>, @FILES[ @{$map{$cmp}} ];
}

# Leftover lines in either file?
print map $empty, map <$_>, @FILES;

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