Anonymous subs are created using sub { }.

Just declare the variable outside of find.

use File::Find;

my $dir;
find(
    sub { $dir = $File::Find::name if ( /hello\.txt/ ); },
    '/tmp/test'
);
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Unfortunately, find will keep on searching for more matches. You can exit prematurely using die

use File::Find;

my $dir;
if (!eval {
    find(
        sub { die "match: $File::Find::name\n" if ( /hello\.txt/ ); },
        '/tmp/test'
    );
    1
}) {
    my $e = $@;
    die $e if !( ($dir) = /^match: (.*)/ );
}
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