This should be close to what you require:

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

open TEST,   '<', 'tests.txt'   or die "Cannot open 'tests.txt' $!";
open OUTPUT, '>', 'output1.txt' or die "Cannot open 'output1.txt' $!";

my $data;
while ( <TEST> ) {
    if ( /SUBSCRIBER/ .. /NATIONAL/ ) {
        $data .= $_;
        next;
        }
    if ( length $data && $data =~ /A1/ ) {
        print OUTPUT $data;
        $data = '';
        }
    }

close TEST;
close OUTPUT;
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