Doesn't the ;?? mean match semi-colon 0 or 1 times and be non-greedy?

Yes, it does, but that's not what you want. You need the .+ to be non-greedy or else disallow semi-colons in your values:

@{$pets} = $1 if $pet_list =~ /$some_variable\:(.*?)(;|\z)/;

# or not allow ';' in values:

@{$pets} = $1 if $pet_list =~ /$some_variable\:([^;]*)(;|\z)/;
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To make the matching more robust, you'll want to do something like this:

@{$pets} = $1 if $pet_list =~ /(?:\A|;)\Q$some_variable\E\:([^;]*)(;|\
+z)/;
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