No need to extend the hash (with 0..9) : 
(my $output = $filter_clean)
    =~ s{ (\D) }{ $filter_map{uc $1} }exg;

[download]
If the aim isn't to separate data and function : 
use strict;
use warnings;

use 5.10.1;

my $filter_clean = 'GaV18';

sub filter2num {
    local $_=shift;
    s/[ABC] /2/igx;
    s/[DEF] /3/igx;
    s/[GHI] /4/igx;
    s/[JKL] /5/igx;
    s/[MNO] /6/igx;
    s/[PQRS]/7/igx;
    s/[TUV] /8/igx;
    s/[WXYZ]/9/igx;
    return $_;
}
say filter2num($filter_clean);

[download]

In reply to Re^2: map and grep or clear code? by brx
in thread map and grep or clear code? by ghenry

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