use strict;
use warnings;
#use utf8;
my $str = "725-275 is an entry - and will be at 423-569 -but- not at 012-457."; 

print "before: $str\n";

$str =~ s/([0-9]{3})-([0-9]{3})/$1\x{2014}$2/g;

print "after: $str\n"

produces:
before: 725-275 is an entry - and will be at 423-569 -but- not at 012-457.
after: 725—275 is an entry - and will be at 423—569 -but- not at 012—457.