One statement: 

$_='A # B # 1 # 2';
print
  join ' # ',
    sub { map @_[$_,$_+@_/2], 0..$#_/2 }->(
      split / # / 
    );

[download]

More readable: 

sub pair { map @_[$_,$_+@_/2], 0..$#_/2 }

$_='A # B # 1 # 2';
print join ' # ', pair split / # /;

[download]

I didn't see the purpose of avoiding split — you didn't say what you didn't like about it — but you could replace
split / # /
with
/(?:(?! # ).)*/sg

In reply to Re: rearranging matched parts of a string while using /g and without using split by ikegami
in thread rearranging matched parts of a string while using /g and without using split by jbullock35

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