if    ( $score >= 90 ) { $grade = 'A'; }
elsif ( $score >= 80 ) { $grade = 'B'; }
#...
else                   { $grade = 'F'; }

##</code><code>##

$grade = ( $score >= 90 ) ? 'A'
       : ( $score >= 80 ) ? 'B'
       : ( $score >= 70 ) ? 'C'
       : ( $score >= 60 ) ? 'D'
       :                    'F';

##</code><code>##

if    ( $score >= 90 ) {
    $grade = 'A';
}
elsif ( $score >= 80 ) {
    $grade = 'B';
}
#...