Two chars off if you put the constant first in the commaprison (good practice anyways :) and get rid of the 'if' statement modifier using '&&':

for(<>){$a=a;$a++while/$a/i;f lt$a&&push@{$_[y///c]},$_}@_=map$_?@$_:(
+),@_;print$_[0]
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Five chars of if you get rid of transforming @_ and grab directly what you want:

for(<>){$a=a;$a++while/$a/i;f lt$a&&push@{$_[y///c]},$_}print+(map$_?@
+$_:(),@_)[0]
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82. I'm sure tybalt89 can do better, though.

update: I wonder why biface isn't a verb proper, meaning treat someone with a biface. The shortest word would then be bifaced which seems fitting to me, huffman-coding anglo-saxonian history.