This works

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

my $time  = "123456.745768";
my $time2 = "123456.745768";
$time =~ s/^(\d{2})(\d{2})(\d{2})\.(\d{2})\d{4}/$1:$2:$3.$4/;
$time2 =~ s/^(\d{2})(\d{2})(\d{2})\.(\d{2})\d*$/$1:$2:$3.$4/;

print "$time\n";
print "$time2\n";
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