The following works for the very limited test cases you've provided. Because I wasn't sure whether by "if there is a "_" after the numbers" you meant "immediately after the numbers", or just "somewhere after the numbers", I invented two more test cases, but I don't know if they represent what your strings actually look like. If you could provide more test cases, we could be of more help.

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;
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Update: Added the other two test cases. Added some more explanation.