The first method favors long lists of strings to undergo the same substitutions. For example, if $str were actually a large array called @str, you would probably favor the first method.

The second method favors lots of substitutions to be performed for a single string.

my $str = "the boy walked the dog";
for ( $str ) {
  s/walked/fed/;
  s/boy/girl/;
  s/dog/Audrey II/;
}
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This method just relies on the fact that the for statement causes $_ to alias $str within the scope of the for statement. And regexp binding (=~) binds to $_ if there is no other variable specified.

You could also do it this way:

my $str = "the boy walked the dog";
my %subs = ( "walked", "fed", "boy", "girl", "dog", "Audrey II" );
foreach ( keys %subs ) {
  $str =~ s/$_/$subs{$_}/;
}
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This method is pretty much opposite of my first example. But it can be handy if you have a lot of substitutions.

Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein