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use strict;
use warnings;
...
2
43 43
END LIST

perl -ni.bak -e 'print unless ( /^Header1$/ .. /^END LIST$/)'  filenam
+e

my $text = do { local $/; <FILE> };
if ($text =~ /^Header1.*?END LIST/m) {
....
}

my $text = do { local $/; <FILE> };
$text =~ s/^Header1.*?END LIST\n?//msg