my @fields = qw/From: Subject:/;  # you can add more if/when you want
my $field_regex = join( "|", @fields );
my @field_lines;

while (<$fh>) {
    push( @field_lines, $_ ) if ( /^(?:$field_regex) / );
    last if @field_lines == @fields;
}
push @field_lines, "";

print join( "\n", sort @field_lines );