use strict;
use warnings;

my $lhs = '(\d*?)-(.*?)-(.*)';
my $rhs = '$1.$2.$3';

while ( my $var = <DATA> ) {
	chomp $var;
	$var =~ s/$lhs/$rhs/ee;
	print $var, "\n";
}

__DATA__
123-456-789
-456-789
789