foreach my $HANDLE (@HANDLES) {

    while(<$HANDLE>) {
        chomp;
        next unless /^PP_START$/ .. /^PP_END$/;

        # process line
      }

}

##</code><code>##

#!/usr/bin/perl

use Modern::Perl '2014';

my @HANDLES = map {
        open my $HANDLE, "<", $_
            or die "Could not open $_: $!\n";
        $HANDLE;
    } @ARGV;

foreach my $HANDLE (@HANDLES) {

    while(<$HANDLE>) {
        chomp;
        next unless /^PP_START$/ .. /^PP_END$/;

        say;

      }
}

##</code><code>##

uninteresting #1
uninteresting #2
uninteresting #3
TX_START
uninteresting #4
uninteresting #5
TX_END
PP_START
interesting #1
interesting #2

##</code><code>##

uninteresting #1
uninteresting #2
uninteresting #3
TX_START
uninteresting #4
TX_END
PP_START
interesting #1
interesting #2
interesting #3
PP_END
uninteresting #5
uninteresting #6
uninteresting #7

##</code><code>##

PP_START
interesting #1
interesting #2
uninteresting #1
uninteresting #2
uninteresting #3
TX_START
uninteresting #4
TX_END
PP_START
interesting #1
interesting #2
interesting #3
PP_END