You could treat @three like it's a queue, primeing it to 3 lines, shifting off the 1st element, and pushing on the next line as the last element:

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

sub getline
{
  my $line = <DATA>;
  return undef unless $line;
  chomp $line;
  return $line;
}

my @three = ( &getline(), &getline(), &getline() );
my $line;
do {
  print join("\t",@three),"\n";
  @three = ( $three[1], $three[2], $line = &getline() );
} while($line);
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