Here is a very crude, brute-force approach. I've no doubt that one of the regex masters here can do it much more efficiently.

use strict;
use warnings;

# I have no idea how unicode might affect this
my $wrongway = 'This is a string with numbers 12345 that must be rever
+sed without reversing the numbers.';

my @tmp;

my @raw = split('',$wrongway);
my @numeric;
my $was_numeric = 0;

for ( @raw ) {
    if ( /\d/ ) {
        push( @numeric, $_ );
        $was_numeric = 1;
        next;
    }
    elsif ( $was_numeric ) {        
        unshift( @tmp, @numeric );
        $was_numeric = 0;
        @numeric = ();
    }
    unshift( @tmp, $_ );
}

my $right_way =join('', @tmp);

print "$wrongway\n";
print $right_way;
print "\n\n";
[download]

Again, this is very ugly but might point the way to how to approach the problem.