Just tell sort that if the RouteDistance is zero, it should use Distance. Note that the result is different to the one you gave. 
#!/usr/bin/perl
use warnings;
use strict;

my @ar = (
          { ID => 3, Distance => 2.3, RouteDistance => 4.3 },
          { ID => 2, Distance => 1.5, RouteDistance => 2.8 },
          { ID => 5, Distance => 2.1, RouteDistance => 0   },
          { ID => 1, Distance => 1.7, RouteDistance => 2.5 },
          );

print $_->{ID}, "\n" for sort {
    ( $a->{RouteDistance} || $a->{Distance} )
          <=>
    ( $b->{RouteDistance} || $b->{Distance} )
} @ar;

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Upate : Fixed precedence.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Sorting an array of hashes with an exeption by choroba
in thread Sorting an array of hashes with an exception by martin_87

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