my @beginnings = ( 
  qr{This is valid</a>},
  qr{[So]+(?:IS|isnt) this},
  qr{start},
);

my @endings = (
  qr{(?:we) Should not [be] [Pp]arsing after},
  qr{either (?:o|f) these [Ll]ines},
  qr{end},
);

my ($re_begin, $re_end, $re_extract);
{
  local $"    = '|';
  $re_begin   = qr{@beginnings};
  $re_end     = qr{@endings};
  $re_extract = qr{$re_begin(.*?)$re_end}s;
}

# is_begin and is_end are no longer needed, but
# you can see how trivial they've become after
# concatenating the regexes with `|'
sub is_begin { shift =~ $re_begin   }
sub is_end   { shift =~ $re_end     }

# extract is equally simple
sub extract  { shift =~ $re_extract }

my $text;
{
  local $/;
  $text = <DATA>;
}

my ($extracted) = extract $text;
print $extracted;

__DATA__
blah blah
blah
start
4
5
6
end
more blah
blah