It is nice to localize the temporary. Sometimes it is also nice to not name the temporary.

In the following, the small trick is...

my $var= "hello, world";
for(  @{[ $var ]}  ) {  # Make a copy
    s/world/earth/;
    say;    # hello, earth
}
say $var;   # hello, world
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...making $_ not be an alias to $var so $var doesn't get modified. Perl 6 gives one explicit control over such (iterate over read-only aliases, read/write aliases, or read/write copies). Perl 5 requires less explicitly targeted techniques. There are other alternatives to @{[ ... ]}, of course.

I don't like the implicit scope that makes this work:

use strict;

my $var= "Hello, World!";
for(  my $copy= $var  ) {
    s/World/Earth/;
    print $_, $/;
}
print $var, $/;
eval 'print $copy, $/; 1'
    or  print '$copy no longer in scope', $/;
__END__
Hello, Earth!
Hello, World!
$copy no longer in scope
[download]

but the implicit scope block is there and the above technique may be clearer (less idiomatic).