from defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ will be checked.

you are getting a true boolean value 1, which means when the option is present, you are just storing the return value of defined($options{o})

try ? :

my $source_file = defined($options{o}) ? "<some filename>" : "as.xcf-d
+ist" ;
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