if ( defined( $q->param( "myinput" ) ) ) {
    print( $q->param( "myinput" ) );
}

##</code><code>##

my $value = $q->param( "myinput" );
print( $value ) if ( $value );

##</code><code>##

my $value = $q->param( "myinput" );
print( $value ) if ( defined( $value ) );