You're almost there with buildRegexp, but you can speedup it further by removing sub call:

sub build_regexp {
  my $expr = join '|', map { "(?:To|is)\:\S+\@$_" } @_;
  eval { $expr = qr/$expr/i  };
  die "aaa!! $@ !!!" if $@;
  $expr;
}
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(note that I'd suggest changing () grouping into (?:) grouping if you're not using $1). then, all you need is

my $writeFh = new FileHandle ">> myout.log";
my $is_subset = build_regexp(@allSubSets);

while (<fh>) {
    next unless /$is_subset/;
    print $writeFh <$fh>;   
}
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