Just a thought I had about the shifting themes - why not tie the scalars to a shifting array (I knew that book on OO Perl would come in useful for something...)
Not sure whether that ugly OO code has any place in a perl poem - what do you think?

# words and concept by alakaboo
# Tied OO implementation of shifting themes
# by Osfameron (hakim@earthling.net)

our @themes = qw( 
   fuel fire 
    flame hot 
     air water 
    earth spirit 
   desire love 
    happiness peace
);


tie $heart, 'Theme', \@themes;
tie $soul, 'Theme', \@themes;

print <<POEM;
from $heart comes $soul
thus the $heart is truly $soul              
for without $heart one cannot have $soul    
and without $soul one returns to the $heart 
one $soul, one $heart                       
no $heart, no $soul                         
POEM

package Theme;
sub TIESCALAR { bless @_[1], @_[0] };
sub FETCH { shift @{@_[0]} };
[download]

Update: On thinking about it, it seems churlish not to mention that that the OO book in question is Damian Conway's excellent Object Oriented Perl.

Cheerio!
Osfameron