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

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

It does, but it only makes that part of the match non-greedy. The (.+) remains greedy.

But if you make that part non-greedy, then you will be asking to capture as little as possible following the ':', that might or might not be followed by a ';'. Which means it will capture just a single character.

A couple of ways to approach the problem:

1. Make capture non-greedy and have a definite terminating condition:/$some_variable\:(.+?)(?:;|$)/

   Where (?:;|$) requires a semicolon or the end-of-string, (prefering the former) to terminate the capture.

2. Or as you know the bit you want to capture cannot contain a ';', reduce the scope of the capture using [^;]+ in place of '.' and omit the terminator.

   /$some_variable\:([^;]+)/
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Either works:

$pet_list = 'dog:boston.terrier;cat:orange.tabby';;

$pets=[]; 
$pet_list =~ /$_\:(.+?)(?:;|$)/ and push @{$pets},$1 for qw[cat dog]; 
print for @{$pets};;
orange.tabby 
boston.terrier

$pets=[]; 
$pet_list =~ /$_\:([^;]+)/ and push @{$pets},$1 for qw[cat dog]; 
print for @{$pets};;
orange.tabby 
boston.terrier
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$array=supersplit(':',';',$mystring);

provided that the (semi)colons are true delimiters.

Cheers,
Jeroen

Couldn't resist the temptation after all those years ;-)