use warnings;
use strict;
use Test::More;

sub subst {
	my $x = shift;
	$x =~ s/\d+[^_:]*\K_/:/;
	return $x;
}

is subst("V12345_name_test"),  "V12345:name_test";
is subst("V12345:name_test"),  "V12345:name_test";
is subst("V12345x_name_test"), "V12345x:name_test";
is subst("V12345x:name_test"), "V12345x:name_test";
# ... more test cases needed here!

done_testing;