GetOptions(
    'f' => \my $force,
    'o' => \my $outdir,
);
$outdir ||= 'thumbnails';

my $needs_remake = My::Dependencies::MTime->new( force => $force );

for my $source_image (@ARGV) {
    if ($needs_remake->([$source_image], ["$outdir/${source_image}_t.jpg"])) {
        ... do work here
    };
};