Athanasius has given the best answer so far, and that's the route you should take, but let's pretend for a moment that there wasn't a suitable module...

The regex approach needs to look for the "From:" and "Subject:" (and other field labels) only when they appear at the beginning of a line, and it can stop reading input as soon as all the desired fields are found. 

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 );

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Note the regex uses the initial anchor character (^) and non-capturing grouping parens. (You could just as well use the simpler capturing parens, without the "?:" -- this would add a slight bit of extra processing, but not enough to worry about here.)

(updated to include parens around the args for the first "push" call, just because I like using parens around function args)

In reply to Re: Matching multiple patterns with regex by graff
in thread Matching multiple patterns with regex by Tiarcon

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