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

# or not allow ';' in values:

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

##</code><code>##

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