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while( <DATA> ) {
    print if /^Header2:/ .. eof;
...
Header2: <headertwo@example.com
Received: from server.cluster1.example.com ([10.20.201.160])
        line 12

my $input = do { local $/ = undef; <DATA> };

if ( $input =~ /^(Header2:.+)/ms ) {
    print $1;
}

my $input;
{
...
    $input = <DATA>;
}
print join '', ( split /^(Header2:)/m, $input, 3 )[ 1, 2 ];

print join '', ( split /^(?=Header2:)/m, $input, 2)[1];

perl -ne 'print if /^Header2:/ .. eof' testdata.txt

...
perl -0777 -pe '$_=join q//,(split /^(Header2:)/m,$_,3)[1,2]' testdata
+.txt

perl -0777 -pe '$_=join q//,(split /^(?=Header2:)/m,$_,2)[1]' testdata
+.txt