If you add $::RD_TRACE = 1; to your script, P::RD will print out a trace of the parse which can help with the debugging. In this case, it shows that the /.+/ms pattern gobbles up "Z 034309201607258" (P::RD can't backtrack into the pattern, it will only backtrack whole rules). Your middle rule should leave the "Z" line at the end. For instance:

#!/usr/bin/perl
use strict; use warnings; use 5.014;

use Parse::RecDescent;
# $::RD_TRACE = 1;

my $text = <<'CONTENT';
Y 034309201607258   1
Q     02751VACANT / CLOSE
G   ....
...

...
T RCRC   0810010
T RDRD   0810010
A 22OP
A 13O
A 12O
P 3472CHSK00010014
Z 034309201607258
CONTENT

my $parser1 = Parse::RecDescent->new(<<'GRAMMAR');
    startrule : HeadRule OtherRule(s) TrailerRule

    HeadRule : /^Y.+$/m
    TrailerRule: /^Z.+$/m

    OtherRule: /^(?![YZ]).*$/m
GRAMMAR

print "Valid data\n" if defined($parser1->startrule($text));
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Also, your parser2 shows why you should almost always end your top-level rule with a /\Z/ pattern so that parsing fails if P::RD doesn't consume the whole string.