Any idea to improve it?

Don't reinvent the wheel. :-)

Any one-liner? ;))

Using Net::Netmask (alt.)...

sub mask_short_to_long { Net::Netmask->new( "0.0.0.0/" . shift )->mask
+ }

# or as a real one-liner! :)

perl -MNet::Netmask -le 'print Net::Netmask->new( "0.0.0.0/" . shift )
+->mask' 24
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    --k.