Thanks to everyone for their wisdom. I agree that s/// isn't in principle the best way to do this, I just was playing with the eval functionality in RHS (right hand side). Hey, if it's good enough for Nathan Torkington (see url at top of comments), it should be good enough for me, right?

But I kept getting comments that there was something wrong with the regex. First, it didn't work with negatives. Then, it didn't work with decimals. Then, it worked *only* with decimals.

Well, I got tired of this back and forth, so here's the same code using Damian Conway's Regexp::Common::Number in the LHS (left hand side). If there's still some issue here, submit a bug report to CPAN. ;)

use strict;
use warnings;
use Regexp::Common;

# From http://prometheus.frii.com/%7Egnat/yapc/2000-stages/slide31.htm
+l
# Fixed a typo in the regex (Nathan forgot to escape the forwardslash.
+)

while (<DATA>) {
  chomp;
  my $celsius = my $fahrenheit = $_;
  #was $fahrenheit =~ s|(\d+)C|($1*9/5)+32 . "F"|ge;
    # But that didn't work with decimals
  #was $fahrenheit =~ s|(\d+\.\d+)C|($1*9/5)+32 . "F"|ge;
    #but that required decimals
  #This is getting ridiculous, so let's just use Damian Conway's Regex
+p::Common
  $fahrenheit =~ s|($RE{num}{real})C|($1*9/5)+32 . "F"|ge;
  print "$celsius is $fahrenheit\n";
}
#Outputs: 
#-2C is 28.4F
#-1.5C is 29.3F
#-1C is 30.2F
#-1.0C is 30.2F
#0C is 32F
#0.0C is 32F
#.1C is 32.18F
#0.1C is 32.18F
#1C is 33.8F
#+1C is 33.8F
#1.5C is 34.7F
#+1.5C is 34.7F
#2C is 35.6F


__DATA__
-2C
-1.5C
-1C
-1.0C
0C
0.0C
.1C
0.1C
1C
+1C
1.5C
+1.5C
2C
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