~$ locale
< ... all set to hu_HU.UTF-8 ... >

[001] ~$ perl -e 'printf(q{%f}, 123.456);'
123.456000

[002] ~$ perl -e 'use locale; printf(q{%f}, 123.456);'
123.456000

[003] ~$ perl -e 'use locale; use POSIX; POSIX::setlocale("LC_ALL", "hu_HU.UTF-8"); printf q{%f}, 123.456;'
123.456000

[004] ~$ perl -e 'use Gtk2 -init; printf(q{%f}, 123.456);'
123,456000