If you write Perl with use strict;, then you’ll find that the one line syntax isn’t valid, even when declared.

With:

my ($newstring = $oldstring) =~ s/foo/bar/;

You get:

Can't declare scalar assignment in "my" at script.pl line 7, near ") =~"
Execution of script.pl aborted due to compilation errors.

Instead, the syntax that you have been using, while a line longer, is the syntactically correct way to do it with use strict;. For me, using use strict; is just a habit now. I do it automatically. Everyone should.

#!/usr/bin/env perl -wT

use strict;

my $oldstring = "foo one foo two foo three";
my $newstring = $oldstring;
$newstring =~ s/foo/bar/g;

print "$oldstring","\n";
print "$newstring","\n";