AFAIK seek doesn't work in pipes. Because pipes are not random access but simple FIFOs.

The best option for you would be to implement a buffer yourself. Simple as that:

my @buffer=();

sub read_a_line {
  if (@buffer) { return shift @buffer; }
  else { my $x= <PIPE>; return $x; }
}

sub push_back_line {
  push @buffer, @_;
}
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