while (<DATA>)

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is equivalent to 

while ($_ = <DATA>)

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which is interpreted as

while (defined($_ = <DATA>))

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So that's how $_ is populated in the original script.

There's another question, though: How $1 is populated. Note that the matching uses =, not =~, so it's equivalent to 

my($regex) = ($_ =~ /This is/g)

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where the parentheses after my enforce the list context on the match, but without a capture group in the regex, there's no way to populate $1.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In reply to Re^3: Split and print hash based on regex by choroba
in thread Split and print hash based on regex by Maire

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