Did a quick benchmark out of curiosity:

KM:     substr($_,0,0,"H")
Ovid:    s/^(.*)$/H$1/
Shendal:  $_ .= "H".$_
turnstep: s/^/H/

Benchmark: timing 100000 iterations of KM, Ovid, Shendal, turnstep...
       KM: -1 wallclock secs ( 0.55 usr +  0.00 sys =  0.55 CPU)
     Ovid:  2 wallclock secs ( 2.20 usr +  0.00 sys =  2.20 CPU)
  Shendal:  0 wallclock secs ( 0.53 usr +  0.00 sys =  0.53 CPU)
 turnstep:  1 wallclock secs ( 0.98 usr +  0.00 sys =  0.98 CPU)
[download]

Cheers,
KM