G'day Zu,

Welcome to the monastery.

I get the feeling that there's some aspect of this that you haven't told us about. For both strings, you have three capture groups but discard $2 in each case. Also, your replacements both have "\nfield2: valueB\n" hard-coded.

However, you've said "The regex below works"; on that basis, this solution is not "slow" for either string.

#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::HiRes qw{time};

my $no_field2 = "field1: valueA\nsome\nlines\nhere\nfield3: valueC\n"
                . "........................................\n" x 1000;
my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n"
                . "........................................\n" x 1000;
my $middle = "\nfield2: valueB\n";

my $re = qr{(^field1:.*?$).*?(^field3:)}ms;

replace_middle($_, $middle, $re) for ($no_field2, $has_field2);

sub replace_middle {
    my ($string, $middle, $re) = @_;

    print '-' x 40;
    print "Start:\n", substr $string, 0, 60;

    my $t0 = time;

    $string =~ s/$re/$1$middle$2/;

    my $t1 = time;

    print "Finish:\n", substr $string, 0, 60;
    print 'Time:   ', $t1 - $t0;
}
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Output:

----------------------------------------
Start:
field1: valueA
some
lines
here
field3: valueC
..............
Finish:
field1: valueA
field2: valueB
field3: valueC
...............
Time:   6.89029693603516e-05
----------------------------------------
Start:
field1: valueA
field2: valueB
field3: valueC
...............
Finish:
field1: valueA
field2: valueB
field3: valueC
...............
Time:   1.62124633789062e-05
[download]