How do I perform a Perl substitution on a string while keeping the original?

Under use strict, say:

(my $new = $original) =~ s/foo/bar/;

instead.

This is the idiom I've always used to get a modified copy of a string without changing the original:

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

In perl 5.14.0 or later, you can use the new /r non-destructive substitution modifier:

my $newstring = $oldstring =~ s/foo/bar/gr; 

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NOTE:
The above solutions work without g too. They also work with any other modifiers.

SEE ALSO:
perldoc perlrequick: Perl regular expressions quick start

The statement:

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

Which is equivalent to:

my $newstring = $oldstring;
$newstring =~ s/foo/bar/g;

Alternatively, as of Perl 5.13.2 you can use /r to do a non destructive substitution:

use 5.013;
#...
my $newstring = $oldstring =~ s/foo/bar/gr;