This program will perform 1000000 string replace operations
and print the number of seconds it takes to complete each.

Testing string replace method #1.
Replace 'H'            --> 'X'               0 second(s)
Replace 'Hello'        --> 'XX'              0 second(s)
Replace 'Hello'        --> ''                1 second(s)
Replace 'Hel'          --> 'XXXXXX'         14 second(s)
Replace 'HelloWorld'   --> 'XXXXXXXXXXXXX'  14 second(s)

Testing string replace method #2.
Replace 'H'            --> 'X'               5 second(s)
Replace 'Hello'        --> 'XX'              2 second(s)
Replace 'Hello'        --> ''                1 second(s)
Replace 'Hel'          --> 'XXXXXX'         16 second(s)
Replace 'HelloWorld'   --> 'XXXXXXXXXXXXX'   1 second(s)

Testing string replace method #3.
Replace 'H'            --> 'X'               1 second(s)
Replace 'Hello'        --> 'XX'            4928 second(s)
Replace 'Hello'        --> ''

##################################################
#PROGRAM STARTS HERE:

use strict;
use warnings;

# See:
# http://www.perlmonks.org/?node_id=98357

my $T;
my $str;
my $find = "";
my $replace = "";
my $REPEAT = 1000000;
my $METHOD = 1;

print "This program will perform $REPEAT string replace operations\n";
print "and print the number of seconds it takes to complete each.\n";

# RUN TESTS

for ($METHOD = 1; $METHOD <= 3; $METHOD++)
{
  print "\nTesting string replace method #$METHOD.\n";
  RunTest("H", "X");
  RunTest("Hello", "XX");
  RunTest("Hello", "");
  RunTest("Hel", "XXXXXX");
  RunTest("HelloWorld", "XXXXXXXXXXXXX");
}

#####################################
sub RunTest
{
  if (@_ != 2) {return;}
  ($find, $replace) = @_;

  $| = 1;   # Disable buffering

  $str = 'HelloWorld' x $REPEAT;

  my $PADDING1 = ' ' x (13 - length($find));
  my $PADDING2 = ' ' x (13 - length($replace));
  print "Replace '$find'$PADDING1--> '$replace'$PADDING2";

  # This method was suggested by tachyon.
  if ($METHOD == 1)
  {
    $find = quotemeta $find;   # escape regex metachars if present
    $T = time;                 # measure time at the start.
    $str =~ s/$find/$replace/g;
    printTime();               # measure time at the end.
  }

  # This method was suggested by harangzsolt33
  if ($METHOD == 2)
  {
    $T = time;                 # measure time at the start.
    $str = join( $replace, split($find, $str) );
    printTime();               # measure time at the end.
  }

  # This method was suggested by davorg.
  if ($METHOD == 3)
  {
    $T = time;                 # measure time at the start.
    my $pos = index($str, $find);
    while ( $pos > -1 )
    {
      substr( $str, $pos, length( $find ), $replace );
      $pos = index( $str, $find, $pos + length( $replace ));
    }
    printTime();               # measure time at the end.
  }
  return;
}
###########################
sub printTime
{
  my $DIFF = (time - $T);
  my $PADDING = ' ' x (3 - length($DIFF));
  print "$PADDING $DIFF second(s)\n";
}
###########################