Probably referring to the magic of: if you use an empty pattern then the last successfull pattern match is re-used. Doesn't show up in your example, but could catch someone unawares:

    #!/usr/bin/perl -w
    use strict;

    # this messes up the intended empty pattern later:
    $_ = 'match';
    /m/;

    my $prefix = 'dog';
    my %hash = map {++$;,$_} (qw(ged house ma matic),'');
    print "Before:\n";
    print "$_ => '$hash{$_}'\n" for keys %hash;

    s//$prefix/for values%hash;   # Add the Prefix...

    print "\nAfter:\n";
    print "$_ => '$hash{$_}'\n" for keys %hash;
    __END__

    output:

    Before:
    1 => 'ged'
    2 => 'house'
    3 => 'ma'
    4 => 'matic'
    5 => ''

    After:
    1 => 'ged'
    2 => 'house'
    3 => 'doga'
    4 => 'dogatic'
    5 => ''
[download]

That could be easily fixed by using s/^/$prefix/ rather than the empty pattern in this case.

Update: just for reference, the relevant documentation is in perlfunc under the /PATTERN/cgimosx operator:

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead.