my $scalar forces the return value of grep to a scalar context, in which case, it returns the number of matches.

my ($scalar) forces the return value of grep to a list context, in which case it returns the first match.

Consider:

use strict;
use warnings;

# MATCHES
{
    my $scalar = grep /\d+/, 1 .. 5;
    print "$scalar\n";
}

{
    my ($scalar) = grep /\d+/, 1 .. 5;
    print "$scalar\n";
}

# NO MATCHES
{
    my $scalar = grep /\d+/, 'a' .. 'z';
    print "$scalar\n";
}

{
    my ($scalar) = grep /\d+/, 'a' .. 'z';
    print( (defined $scalar) ? "$scalar\n" : "undefined\n" );
}

__END__

5
1
0
undefined
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See also: List value constructors

Update: added 'no matches' to example