If you are trying to match an exact substring then there is usually little point in using regex for that. I would use index instead.

use strict;
use warnings;
use Test::More tests => 1;

my $corpus = q{foo<span class="Trsdu(0.3s) Fw(b) Fz(36px) Mb(-4px) D(i
+b)" data-reactid="33">bar};
my $lookfor = q{<span class="Trsdu(0.3s) Fw(b) Fz(36px) Mb(-4px) D(ib)
+" data-reactid="33">};

my $res = index ($corpus, $lookfor) > -1 ? 1 : 0;
ok $res, 'Substring found';
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Update: Replacing the ternary with a simple increment gives us more info in $res with the same testing ability. Adding a negative test too gives the better example:

use strict;
use warnings;
use Test::More tests => 2;

my $corpus = q{foo<span class="Trsdu(0.3s) Fw(b) Fz(36px) Mb(-4px) D(i
+b)" data-reactid="33">bar};
my $lookfor = q{<span class="Trsdu(0.3s) Fw(b) Fz(36px) Mb(-4px) D(ib)
+" data-reactid="33">};

my $res = index ($corpus, $lookfor) + 1;
ok $res, 'Substring found';

$corpus = 'Something else';
$res = index ($corpus, $lookfor) + 1;
ok !$res, 'Substring not found';
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