You can avoid modifying $_ within the sub, by doing the initialization of $hash_ref in a BEGIN block:

{
  my $hash_ref;
  BEGIN {
    while (<DATA>){
      chomp;
      my ($str, $num) = split /\|/;
      $hash_ref->{$str} = $num;
    }
  }
  sub get_number{
    my ($num) = @_;
    return $hash_ref->{$num};
  }
}
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Update: In light of hv's comments below, I conclude that, in this case at least, saving the while (<DATA>) is not worth all the fuss and worry over $_. Hence in the revision below, the loop does not use $_.

{
  my $hash_ref;
  BEGIN {
    while (defined (my $line = <DATA>)){
      chomp $line;
      my ($str, $num) = split /\|/, $line;
      $hash_ref->{$str} = $num;
    }
  }

  sub get_number{
    my ($num) = @_;
    return $hash_ref->{$num};
  }
}
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