If you modify your code to

 echo -e 'zero\none\ntwo\n\n    three' | perl -0777 -pe 's{\s*(one\ntwo)\s*?( *)}{\n\n<begin block>\n\n$1\n\n<end block>\n\nx$2x}'

so you have a clear delimiter around your second match, your get: 

zero

<begin block>

one
two

<end block>

xx

    three

[download]

By swapping your space matching to non-greedy, you are no longer consuming the newlines preceding 'three'. You can get your expected result by using the multiline modifier (see Modifiers in perlre) combined with a line start metacharacter ^;

echo -e 'zero\none\ntwo\n\n    three' | perl -0777 -pe 's{\s*(one\ntwo)\s*^( *)}{\n\n<begin block>\n\n$1\n\n<end block>\n\n$2}m'

yields 

zero

<begin block>

one
two

<end block>

    three

[download]

In reply to Re: matching and mysterious captures by kennethk
in thread matching and mysterious captures by Allasso

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