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perl -e "$_='aaaaaaaaaa'; substr($_,0,-4)='X' x (length($_) -4); print
+"

perl -e "$_='1234567890'; substr($_,0,-4, 'X' x (length($_) - 4)); pri
+nt"

use Benchmark;
...
 Substring:  6 wallclock secs ( 5.19 usr +  0.00 sys =  5.19 CPU) @ 31
+6184.66/s (n=1640366)
   fourarg:  6 wallclock secs ( 5.10 usr +  0.00 sys =  5.10 CPU) @ 37
+0755.39/s (n=1890111)
  mysubstr:  6 wallclock secs ( 5.39 usr +  0.00 sys =  5.39 CPU) @ 27
+1099.83/s (n=1460957)