Yes, it isn't that hard to write one. But you missed one aspect in your version: filter { s/a/b/ } @list shouldn't modify the original @list.

Let me go dig up my version...

Update: Make that two features. (: I also think that 'filter' in a scalar context should return the concatenated values (a count is pretty useless):

package filter;

use strict;
require Exporter;
{ my $nowarn= *import }
*import= \&Exporter::import;
use vars qw( @EXPORT );
@EXPORT= qw( filter );

sub filter(&@) {
    my( $code, @vals )= @_;
    # local( $_ ); # Done by the loop.
    for(  @vals  ) {
        $code->();
    }
    wantarray ? @vals : join "", @vals;
}

1;
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Though I've only tested this a little.

        - tye (filtering shouldn't be straining)