In an eval evironment in which  @_ was, e.g.,
    @_ = ('searchPattern', 'replacementString', 'regexModifiers');
(presumably, the eval statement is called in the context of a function to which arguments were passed via @_), the eval would return a code reference equivalent to
    sub { s/searchPattern/replacementString/regexModifiers for @_ }
which would allow one to iterate over a list of strings and do substitutions on each (non-literal!) string: 

my $x = 'some';
my $y = 'strings';
my $z = 'here';

$sub->($x, $y, $z);

[download]

The only benefit that I can see of using the string eval is that the regex | substitution operator modifiers  /g /e /r and perhaps a couple others cannot be "passed" in any other way. If not for these modifiers, one could simply write something like (again, assuming this is in a function in which  @_ held search/replace strings)
    my $sub = sub { s/$_->[0]/$_->[1]/xmsg for @_ };
    ...
    $sub->($x, $y, $z);
(note the literal modifier group) and get the same result. In this example, the modifiers other than  /g could be passed in a string.

Update: E.g., 

c:\@Work\Perl\monks>perl -wMstrict -le
"S('(?xmsi) foo', 'bar', 'g');
 ;;
 sub S {
   my $sub = eval qq{ sub { s/$_[0]/$_[1]/$_[2] for \@_ } };
   ;;
   my $x = 'foo';
   my $y = 'Foo fOo foO';
   my $z = 'FOO';
   ;;
   $sub->($x, $y, $z);
   print qq{x '$x'  y '$y'  z '$z'};
   }
"
x 'bar'  y 'bar bar bar'  z 'bar'

[download]
And yes, this is a trick for a completely trusted environment. But then: "Trust, but verify!"

Give a man a fish:  <%-{-{-{-<

In reply to Re^3: Precompiling substitution regex by AnomalousMonk
in thread Precompiling substitution regex by FreakyGreenLeaky

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