Compress() is easily defined in terms of a generic adjacent pairing routine:

#! perl -slw
use strict;

sub adjacentPairs (&@) {
    my $code = shift;
    map {
        local( $a, $b ) = @_[ $_-1, $_ ];
        $code->();
    } 1 .. @_;
}


print adjacentPairs{
    defined $b && $a eq $b ? () : $a
} qw[a a a b c c a a d e e e e];

__END__
C:\test>compress
abcade
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