This works on the input you provided. It looks for the 1st digit followed by an underscore and replaces _ with :. Refer to s/// 
use warnings;
use strict;

while (<DATA>) {
    s/(\d)_/$1:/;
    print;
}

__DATA__
V12345:name_test
V12345_name_test

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In reply to Re: replace first occurance if it doesn't already match by toolic
in thread replace first occurance if it doesn't already match by ddrew78

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