use Modern::Perl;

my $STX = 'a';
my $ETX = 'z';

my $re = qr{ [$STX] (?: (?<match>[^$ETX]{80}) | (?:(?<match>[^$ETX]{0,79}) [$ETX])) }x;

$_='a12345678za1234567890123456789012345678901234567'.
'890123456789012345678901234567890123456789012345678901234567890z';

say $+{match} while (/$re/g);