#!/usr/bin/perl

my $datafile = "sample.pm";

open( I, $datafile ) or die "open failed on $datafile: $!";

my @editable;
my ( $bgn, $end ) = ( '', '' );
my $save = \$bgn;

while (<I>) {
    if ( defined( $save )) {
        $$save .= $_;
        $save = undef if ( /^\#\s+__START_CONFIG__/ );
    }
    elsif ( /^\#\s+__END_CONFIG__/ ) {
        $end .= $_;
        $save = \$end;
    }
    else {
        push @editable, $_;
    }
}

# $bgn holds all input up to and including "# __START_CONFIG__"
# $end holds "# __END_CONFIG__ and all input that follows it
# @editable holds all lines between $bgn and $end