$time2 =~ s/^(\d{2})(\d{2})(\d{2})\.(\d{2}).+?/$1:$2:$3.$4/;

##</code><code>##

#!/usr/bin/perl
#
#
use strict;
use warnings;

my $time2 = "123456.745768";
$time2 =~ s/^(\d{2})(\d{2})(\d{2})\.(\d{2}).+?/$1:$2:$3.$4/;
print "$time2\n";
__END__
12:34:56.74768

##</code><code>##

s/^(\d{2})(\d{2})(\d{2})\.(\d{2})\d*/$1:$2:$3.$4/;