This wouldn't be an issue at all if you used the standard module Getopt::Long.

Anyway,

my $source_file = defined($options{o}) ? $options{o} : 'as.xcf-dist';
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or

my $source_file = 'as.xcf-dist';
$source_file = $options{o} if defined($options{o});
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or

my $source_file = $options{o};
$source_file = 'as.xcf-dist' if !defined($options{o});
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or

sub defined_or { defined($_[0]) ? $_[0] : $_[1] }
my $source_file = defined_or( $options{o}, 'as.xcf-dist' );
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or

# Requires 5.10
my $source_file = $options{o} // 'as.xcf-dist';
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