Here's a variation on your theme:

##### MatchVars.pm #####
package MatchVars;
use strict;
use warnings;

sub tie {
    my %args = @_;
    my $string = delete $args{string} || return;
    while(my($k,$v) = each %args){
        tie $$v, $k, $string
    }    
}

package match;
sub TIESCALAR {bless [$_[1]],$_[0]}
sub FETCH {substr(${$_[0][0]}, $-[0], $+[0] - $-[0] )}
    
package pre;
use base "match";
sub FETCH {substr(${$_[0][0]},0, $-[0] )}

package post;
use base "match";
sub FETCH {substr(${$_[0][0]}, $+[0] )}

1;
__END__

##### match_test.pl #####
#!/usr/bin/perl -w
use strict;
use warnings;
use MatchVars;

my ( $match, $pre, $post, $string );
MatchVars::tie( string => \$string,
                pre    => \$pre,
                match  => \$match,
                post   => \$post
) || die "Can't tie matchvars";

$string = "This is a contrived test string...";
if ( $string =~ /contrived/) {
        print $pre,"\n";
        print $match, "\n";
        print $post, "\n";
}

$string = 'and something else entirely';
if ( $string =~ m/thing/ ) {
        print $pre,"\n";
        print $match, "\n";
        print $post, "\n";
}
__END__
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