gistfile1.6pl

#!/usr/bin/env perl6
use v6;

sub infix:<joins> ($a,$b) {
    $a.substr(*-1,1) eq $b.substr(0,1) ?? "$a $b" !! Empty;
}

say <the that a>           Xjoins
    <elephant thing>       Xjoins
    <walked treaded grows> Xjoins
    <slowly quickly>;

gistfile2.6pl

#!/usr/bin/env perl6
use v6;

sub infix:<lf> ($a,$b) {
    next unless try $a.substr(*-1,1) eq $b.substr(0,1);
    "$a $b";
}
 
multi dethunk(Callable $x) { try take $x() }
multi dethunk(     Any $x) {     take $x   }
 
sub amb (*@c) { gather @c».&dethunk }
 
say first *, do
    amb(<the that a>, { })      Xlf
    amb(<frog elephant thing>)  Xlf
    amb(<walked treaded grows>) Xlf
    amb(<slowly quickly>, { });