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

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

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