It makes no sense to use it on scalars.

    @foo = (
        ...
        ( $include_bar ? 'bar' : () )
        ...
    );
[download]

    @foo = (
        ...
        grep( $include_bar, 'bar' )
        ...
    );
[download]

Ooh, nice! That's eminently readable. Much more likely to survive a code review than aristotle's ex-bang-bang operator.

Rather than

my @part = (
    'http://example.net/app',
    ( 'admin'  ) x!! $is_admin_link,
    ( $subsite ) x!! defined $subsite,
    $mode,
    ( $id      ) x!! defined $id,
    ( $submode ) x!! defined $submode,
);
[download]

we would now have

my @part = (
    'http://example.net/app',
    grep( $is_admin_link.    'admin' ),
    grep( defined($subsite), $subsite ),
    $mode,
    grep( defined($id),      $id      ),
    grep( defined($submode), $submode ),
);
[download]

I like it.

• another intruder with the mooring in the heart of the Perl

What is the difference?

[grep] searches arrays or lists.

I don't think so. Do you have any reason to believe grep knows anything about arrays?

>perl -le"@a = qw( a b c d ); @b = grep { push(@a,'!') if !$i++; 1 } @
+a; print @b"
abcd
[download]

Contrast with for which does:

>perl -le"@a = qw( a b c d ); for (@a) { push(@a,'!') if !$i++; push @
+b, $_ } print @b"
abcd!
[download]

Grep could have been called "filter" in array context.
Most common form is @output = grep{anon_sub_returning_True_False}@input;
This code:

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

my @a = qw( a b c d ); 
my $i=0;
my @b = grep { push(@a,'!') if !$i++; 1 } @a; 
print "@b\n";
print "@a\n";
__END__
output:
a b c d
a b c d !
[download]

Means take each item in @a and put it into the anon grep subroutine. If that subroutine returns true, then move that item to @b. Absent an explicit return statement, the return value of a Perl sub is the last statement. In this case it is "1", or always true. So this is the same as: @a=@b; as far as grep{} is concerned. This push statement gets executed but has no bearing upon the action of grep{}.

So, YES grep{} is meant to process/filter lists! I would argue that putting this push statement inside the grep is a weird thing to do.

grep does have a scalar context also! This is useful in some cases, here this is just a "dumb" example.

my $num = grep{1}@a;
print $num; ##prints 5 (num of elements in @a) ie, abcd!
[download]

But this can be used to say count number of matches in @a.
$num = grep {/some_regex/}@a;
But of course most common use would be @a=grep{/regex/}@b; which moves any item from @b which matches regex to @a.

Update:Perl will not allow the input list to be modified within the grep{} subroutine. This could create a circular endless loop and that just won't happen.

So, YES grep{} is meant to process/filter lists! I would argue that putting this push statement inside the grep is a weird thing to do.

Obviously processes/filters lists. Obviously a weird thing to do. Jethro said grep also works on arrays, not just lists, and I was demonstrating that it only works on lists.