There must be something else going on. I tested your second code and it works for me:

#!/usr/bin/perl
use warnings;
use strict;

my $content = 'Fields of activity:

Legal basis:';

my $d_fieldofactivity = 'missing';
if ($content =~ /Fields \s of \s activity: (.*) Legal/xs) {
    $d_fieldofactivity = $1;
}

print "(( $d_fieldofactivity ))\n";
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Output:

(( 

 ))
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