if ( my $lost = @snapshot_listing - keys %snapshot_roster ) {
    warn sprintf( "There were %d bad entries in %d input records\n",
            $lost, scalar @snapshot_listing );
}

##</code><code>##

my @bad_entries;
sub nfg { push @bad_entries, $_; return }

my %snapshot_roster = map {
    m[(snap-\S+).*+-(\d+)$] ? ($1,$2) : nfg()
} @snapshot_listing;

if ( @bad_entries ) { warn "oh well... win some, lose some.\n" }