This is completely missing timestamped checks. The database can be timestamped so you know whether to look deeper or not. Same with the tables. Same with the row. Then the values are marked with sequence numbers so you know which ones have been updated and which ones haven't. This gives you the possibility of handling a field-level change to the same row in multiple copies.

You'd need to make notes somewhere about stuff that'd been deleted. More timestamps.

Further, using timestamps allows a person to work on only the changed portion. This is of prime importance when the data set is too large to do a complete compare in a reasonable way or when it is too large to comfortably hold in memory.

So what's your method for comparing? Something approximating foreach ( @src ) { $tgt = fetch_this( $_->id ); update() if $_ ne $tgt } is clearly not goot enough for anything except very small data sets or people with memory and bandwidth to burn.