#!/usr/bin/perl

$a = "Hello, this is a test.";

# attempt to substitute "was" for "is", case sensitive
$a =~ s/is/was/;
print "$a\n";
# Hello, thwas is a test.

# try again...
$a = "Hello, this is a test.";

# attempt to substitute " was" for " is", case sensitive
$a =~ s/ is/ was/;
print "$a\n";
# Hello, this was a test.

# global specifier "g" afterward will do all not just 1st one
$a = "hi hi hi";
$a =~ s/hi/ho/g;
print "$a\n";
# ho ho ho