You can use the fact that all lines of the paragraphs are indented, but the first line.

my @p;

while (<DATA>)
{
  push @p, [],
    if /^Window/;
    
  push @{$p[-1]}, $_;
}
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or maybe you want a hash and use the window handles as keys:

my %h;

while (<DATA>)
{
  $key = $1,
  next
    if /^Window ([^:]+)/;

  push @{$h{$key}}, $_;
}
[download]