Well, map does come to mind... ;-)

use strict;

# define hash keys that need conversion
# (others will be unchanged)
my %map_foo_to_bar = (
    gearbox    => 'gearbox1',
    drivetrain => 'the_drivetrain',
    smokestack => 'my_smokestack',
    junkyard   => '_junkyard',
);

# our hash data
my %foo = (gearbox        => 'five-speed', 
           drivetrain     => 'automatic', 
           something_else => 'entirely');

# perform the mapping (wrapped in an anonymous hashref)
my $new_name;
my $bar = { 
  map 
    { defined ($new_name = $map_foo_to_bar{$_}) ? 
      ($new_name => $foo{$_}) : 
      ($_        => $foo{$_})
    } 
    (keys %foo) 
  };
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Then from Data::Dumper:

$VAR1 = {
          'the_drivetrain' => 'automatic',
          'something_else' => 'entirely',
          'gearbox1' => 'five-speed'
        };
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I've come to think of map a potential solution whenever I want to apply the same operation to every element of an array or hash. In this case the "same operation" just happens to be a conditional -- if we have a new name defined for a given hash key, then use it, otherwise just keep the original hash key.