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use utf8;
my $str = "725-275 is an entry - and will be at 423-569 -but- not at 0
+12-457."; 
$str =~ s/(\d{3})-(\d{3})/\1\_\2/g;

print "$str\n"

725_275 is an entry - and will be at 423_569 -but- not at 012_457.