Shouldn't the substitution of $time2 work? IE -- see two digits after the '.' and then leave it at that?

No ;-), because you are substituting the matching part, and the trailing digits do not match, they are therefore left alone.

I even tried

$time2 =~ s/^(\d{2})(\d{2})(\d{2})\.(\d{2}).+?/$1:$2:$3.$4/;
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but got the same (bad) results.

I didn't:

#!/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
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Notice the missing five? .+? is non greedy and matches only what it has to, in this case one single character.

take away the ?, and it does, what you want; I'd change the . (dot) to \d and the + to a * though, resulting in

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